1. Field of the Invention
The present invention generally relates to a magnetic disk device including a positioning control mechanism for positioning a head at a target position on a magnetic disk, and a head-position control method.
2. Description of the Related Art
In a head-positioning control system for a magnetic disk device, a microcomputer is generally used to configure a digital control system. In the digital control system, a control command is calculated in the microprocessor from discretely obtained position information for a head, and the control command calculated is provided to a drive of an actuator through a digital-to-analog (D/A) converter.
The actuator generally has mechanical resonance in a high-frequency band, and therefore, to move the head to a target position on the magnetic disk at high velocity with low vibration and low noise, it is extremely important to generate a feedforward control input, which does not excite the mechanical resonance. To move the head only for a short distance at high velocity, a technique as follows can be considered. The technique is such that a feedforward control input for the actuator so as not to excite the mechanical resonance and a target position command for a feedback control system are previously calculated by using an optimization technique to be stored as a table on memory, and the feedforward control input for the actuator and the target position command for the feedback control system are determined by referring to the table upon positioning the head.
However, in the technique for head-positioning control, the feedforward control input to the actuator and the target position command to the feedback control system for all the seek distances need to be held in the table. Therefore, it is impossible to implement this technique because the memory capacity of the microprocessor is limited.
Consequently, if a long-distance seek is required, it is necessary to successively calculate the feedforward control input for the actuator and the target position command. For example, JP-A H9-073618 (KOKAI) discloses a technology of holding a model control system for an actuator in a positioning control system and causing a model velocity to follow a target velocity curve, to output a control command for the model control system and a model position, which are the feedforward control for the actuator and the target position command, to the feedback control system.
In such a conventional technology as above, the control command for the model control system and the model position are output, as the feedforward control input for the actuator and the target position command, to the feedback control system. Therefore, when a short-distance seek operation is required, the feedforward control input for the actuator and the target position command cannot previously be calculated by using the optimization technique. Thus, the configuration of the control system that causes the model velocity to follow the target velocity curve becomes particularly important.
In the conventional technology, however, for example, paragraph 0044 in JP-A H9-073618 (KOKAI) describes “a compensation element 35 is an element which multiplies a velocity error of a difference element 34 by a gain of a constant value K”. As described above, because the model velocity is caused to follow the target velocity, only a simple velocity feedback control system is provided.
Therefore, when the target velocity is increased, the model velocity cannot follow the target velocity, overshoot thereby occurs at a model position.
Further, to improve the following capability of the model velocity to the target velocity, it can also be considered to set a velocity feedback gain to a high value. In this case, however, the velocity is abruptly switched to deceleration as soon as the model velocity reaches the target velocity, and a feedforward control command with abrupt switching from acceleration to deceleration is provided to the actuator. As a result, excitation of the mechanical resonance cannot be suppressed.